Expression of fibroblast growth factor-5 by bovine choroidal endothelial cells in vitro

To demonstrate the expression of fibroblast growth factor-5 (FGF-5) by bovine choroidal microvascular endothelial (BCME) cells and to investigate its possible role as an autocrine mitogen in these cells.

Expression of FGF-5 by BCME cells was studied by a combination of Northern and Western blot analyses. Total RNA was isolated from BCME cultures at passages 5 through 8 and analyzed by Northern blot analysis for the presence of FGF-5 transcripts, using a 1-kb human complemetary DNA. Slot-blot analysis was performed to determine possible cross-reactivity between this probe and acidic and basic FGFs of human and bovine species. A previously characterized antibody directed against the aminoterminus of the human FGF-5 sequence was used in Western blot analyses to identify immunoreactive proteins released by BCME cells into the medium. Finally, the mitogenic activity of human recombinant FGF-5 on a variety of cell types was evaluated, using a cellular proliferation assay.

Northern blot analysis provided evidence for the expression of two major FGF-5 transcripts at 4 kb and 3 kb and two minor transcripts at 2.2 kb and 1.7 kb. A single immunoreactive protein with a molecular weight of 34 kDa was identified by Western blot analysis of conditioned media. In cellular proliferation assays, human recombinant FGF-5 was not mitogenic in BCME cells but exhibited an approximate ED50 of 1.8 to 3.7 nM in BALB/c3T3 fibroblasts. This ED50 was within the range reported by the manufacturer, using a thymidine incorporation assay and a similar embryonic fibroblast cell line. Fibroblast growth factor-5 also stimulated proliferation of human retinal pigment epithelial cells.

Bovine choroidal microvascular endothelial cells exhibit expression in vitro of FGF-5 at the messenger RNA and protein levels. Perivascular and endothelial cell staining for FGF-5 seen previously in choroidal neovascular membranes may therefore arise from expression by choroidal endothelial cells. Because nonglycosylated recombinant FGF-5 does not appear to be a mitogen in BCME cells in vitro, it is reasonable to question its role as an autocrine mitogen in vivo. Fibroblast growth factor-5 may instead be serving paracrine roles in the stimulation of fibroblasts and retinal pigment epithelial cells during the formation of choroidal neovascular membranes. Studies with fully glycosylated recombinant FGF-5 will be required, however, to assess the biologic activity of this member of the FGF gene family.